16 research outputs found
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Generalized Kasha's Model: T-Dependent Spectroscopy Reveals Short-Range Structures of 2D Excitonic Systems
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Long-range energy transport in photosystem II.
We simulate the long-range inter-complex electronic energy transfer in photosystem II-from the antenna complex, via a core complex, to the reaction center-using a non-Markovian (ZOFE) quantum master equation description that allows the electronic coherence involved in the energy transfer to be explicitly included at all length scales. This allows us to identify all locations where coherence is manifested and to further identify the pathways of the energy transfer in the full network of coupled chromophores using a description based on excitation probability currents. We investigate how the energy transfer depends on the initial excitation-localized, coherent initial excitation versus delocalized, incoherent initial excitation-and find that the overall energy transfer is remarkably robust with respect to such strong variations of the initial condition. To explore the importance of vibrationally enhanced transfer and to address the question of optimization in the system parameters, we systematically vary the strength of the coupling between the electronic and the vibrational degrees of freedom. We find that the natural parameters lie in a (broad) region that enables optimal transfer efficiency and that the overall long-range energy transfer on a ns time scale appears to be very robust with respect to variations in the vibronic coupling of up to an order of magnitude. Nevertheless, vibrationally enhanced transfer appears to be crucial to obtain a high transfer efficiency, with the latter falling sharply for couplings outside the optimal range. Comparison of our full quantum simulations to results obtained with a "classical" rate equation based on a modified-Redfield/generalized-Förster description previously used to simulate energy transfer dynamics in the entire photosystem II complex shows good agreement for the overall time scales of excitation energy transport
Multiscale model of light harvesting by photosystem II in plants.
The first step of photosynthesis in plants is the absorption of sunlight by pigments in the antenna complexes of photosystem II (PSII), followed by transfer of the nascent excitation energy to the reaction centers, where long-term storage as chemical energy is initiated. Quantum mechanical mechanisms must be invoked to explain the transport of excitation within individual antenna. However, it is unclear how these mechanisms influence transfer across assemblies of antenna and thus the photochemical yield at reaction centers in the functional thylakoid membrane. Here, we model light harvesting at the several-hundred-nanometer scale of the PSII membrane, while preserving the dominant quantum effects previously observed in individual complexes. We show that excitation moves diffusively through the antenna with a diffusion length of 50 nm until it reaches a reaction center, where charge separation serves as an energetic trap. The diffusion length is a single parameter that incorporates the enhancing effect of excited state delocalization on individual rates of energy transfer as well as the complex kinetics that arise due to energy transfer and loss by decay to the ground state. The diffusion length determines PSII's high quantum efficiency in ideal conditions, as well as how it is altered by the membrane morphology and the closure of reaction centers. We anticipate that the model will be useful in resolving the nonphotochemical quenching mechanisms that PSII employs in conditions of high light stress
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Generalized Kasha's Scheme for Classifying Two-Dimensional Excitonic Molecular Aggregates: Temperature Dependent Absorption Peak Frequency Shift
We propose a generalized theoretical framework for classifying
two-dimensional (2D) excitonic molecular aggregates based on an analysis of
temperature dependent spectra. In addition to the monomer-aggregate absorption
peak shift, which defines the conventional J- and H-aggregates, we incorporate
the peak shift associated with increasing temperature as a measure to
characterize the exciton band structure. First we show that there is a
one-to-one correspondence between the monomer-aggregate and the T-dependent
peak shifts for Kasha's well-established model of 1D aggregates, where
J-aggregates exhibit further redshift upon increasing temperature and
H-aggregates exhibit further blueshift. On the contrary, 2D aggregate
structures are capable of supporting the two other combinations: blueshifting
J-aggregates and redshifting H-aggregates, owing to their more complex exciton
band structures. Secondly, using spectral lineshape theory, the T-dependent
shift is associated with the relative abundance of states on each side of the
bright state. We further establish that the density of states can be connected
to the microscopic packing condition leading to these four classes of
aggregates by separately considering the short and long-range contribution to
the excitonic couplings. In particular the T-dependent shift is shown to be an
unambiguous signature for the sign of net short-range couplings: Aggregates
with net negative (positive) short-range couplings redshift (blueshift) with
increasing temperature. Lastly, comparison with experiments shows that our
theory can be utilized to quantitatively account for the observed but
previously unexplained T-dependent absorption lineshapes. Thus, our work
provides a firm ground for elucidating the structure-function relationships for
molecular aggregates and is fully compatible with existing experimental and
theoretical structure characterization tools
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Models and mechanisms of the rapidly reversible regulation of photosynthetic light harvesting.
The rapid response of photosynthetic organisms to fluctuations in ambient light intensity is incompletely understood at both the molecular and membrane levels. In this review, we describe research from our group over a 10-year period aimed at identifying the photophysical mechanisms used by plants, algae and mosses to control the efficiency of light harvesting by photosystem II on the seconds-to-minutes time scale. To complement the spectroscopic data, we describe three models capable of describing the measured response at a quantitative level. The review attempts to provide an integrated view that has emerged from our work, and briefly looks forward to future experimental and modelling efforts that will refine and expand our understanding of a process that significantly influences crop yields